1. Field of the Invention
The present invention relates to a device for testing the adhesive qualities of a coating, and more particularly, to a pneumatic or hydraulic device for carrying out tests, such as measuring the adhesive properties of a coating on a substrate, or tensile testing of materials that possess little elasticity.
2. Discussion of Related Art
Devices for testing the adhesion of a coating, e.g., paint or adhesives, to an underlying surface have been known for many years. The accurate testing of the adhesion of a material has importance to many industries. It is therefore important to have a device that provides accurate and consistent results.
Many of the prior art devices utilize a dolly that is fixed by an adhesive to a coating on an underlying substrate to test the adhesion strength between the substrate and the applied coating. A frame engages the dolly and applies a force to the dolly that is perpendicular to the substrate. The amount of force that is applied to the dolly to make the coating separate from the substrate corresponds to the adhesive strength of the coating to the substrate.
Several of the prior art devices engage the dolly with a structure that a has a plurality of legs that surround the dolly. The legs engage the coating on which the dolly is secured and apply a counteracting force to the coating. A problem with such devices is if the force is not applied evenly to all of the legs, the applied tensile force may be applied unevenly to the dolly. If the force is not applied evenly to the dolly, the coating between the dolly and the substrate may begin to yield at only one edge of the dolly, thus providing an inaccurate reading.
To overcome this problem, some of the prior art devices apply means for adjusting the length of the legs supporting the refracting device. Other devices rely on a pneumatic or hydraulic system that applies a pressure to all of the legs uniformly. See, e.g., U.S. Pat. No. 5,361,639.
However, such devices do not provide a self-centering device that engages the test dolly. Accordingly, even if the pressure is applied evenly to all of the legs, and if all of the legs are adjusted to the same length, if the dolly is skewed at a slight angle, the adhesion tester still may apply a force such that the coating tends to separate from the substrate at one edge thereof.
Other prior art devices provide an opening in the center of the doily through which a pin is used to apply a force against the coating. A problem with such a structure is that the dolly does not have an uninterrupted surface area that is adhered to the coating.